I don't know why I would want to use a feature ID in a relation, since I'd be using parameters, or dimensions, etc. which are already unique in a part or assembly.
Parameters can exist in many different places in a model. It can be very helpful to be able to refer to these feature level parameters without having to map all of them back to the top level parameters first.
So this could mean I have parameters defined within the scope of a feature? I've used relations within a feature sketch, but I don't see any way to define parameters within a feature. The option is "grayed out" when I'm defining the feature.
The dimensions in a feature are defined as "SD1", SD2" while within the feature sketch, but they get assigned a unique dimension name automatically.